Year	Years
1970	4.02
1971	4.11
1972	4.2
1973	4.29
1974	4.39
1975	4.48
1976	4.58
1977	4.68
1978	4.78
1979	4.88
1980	4.99
1981	5.09
1982	5.2
1983	5.3
1984	5.41
1985	5.52
1986	5.63
1987	5.75
1988	5.86
1989	5.98
1990	6.09
1991	6.21
1992	6.33
1993	6.45
1994	6.57
1995	6.7
1996	6.82
1997	6.95
1998	7.07
1999	7.2
2000	7.33
2001	7.46
2002	7.59
2003	7.72
2004	7.85
2005	7.99
2006	8.12
2007	8.26
2008	8.4
2009	8.53
2010	8.67
2011	8.81
2012	8.95
2013	9.09
2014	9.23
2015	9.37